Updated design decision file in doc.

doc/design_decisions.md

@@ -1,2 +1,16 @@
 # Design Decisions
+- Code structure
+
+App creates a race instance which can:
+instantiate a file parser to extract race setting and team information;
+creates and passes legs and teams/boats into event generator to create events;
+runs a race and iterates all events that returned from the generator;
+prints out event details, including time, involved boats and legs.
+
+- Configuration file
+
+We decided to store the team information including team names and boat velocity, as well as race configuration setting in external file.
+
+To read external files, "Json-simple" library has been used to parse information. 
+By using this library, we did not have to write our json parser and benefited from the flexibility of json files.